A-To-d converter of the successive-approximation type

ABSTRACT

An integrated-circuit analog-to-digital converter of the successive-approximation type formed on a single monolithic chip. The converter is made by a diffusion process wherein certain portions of the chip are formed with normal-mode linear transistors, and other portions are formed with inverted mode I 2  L transistors. The normal-mode transistors provide a switchable current-source DAC, a set of three-state output buffers, and a comparator. The inverted mode transistors provide an internal clock and successive-approximation control circuitry for the DAC. The chip also includes a voltage reference to provide for absolute analog-to-digital conversions.

This is a division of application Ser. No. 931,960 filed Aug. 8, 1978U.S. Pat. No. 4,400,689 which in turn is a continuation in part of Ser.No. 785,322, filed Apr. 7, 1977 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to analog-to-digital converters adapted toreceive an electrical analog input signal and to produce a correspondingelectrical digital output signal. More particularly, this inventionrelates to a successive-approximation A/D converter constructed inintegrated circuit (IC) form.

2. Description of the Prior Art

Analog-to-digital converters of various types have been in use for manyyears, typically employed for converting analog measurements and thelike into corresponding digital signals for processing by high-speeddigital computers. For certain applications, there has been considerableuse of converters incorporating electronic ramp-signal integrators withclock-pulse timing devices for producing a digital count correspondingto the magnitude of an analog signal. A converter of the latter typehaving important advantageous features is disclosed in U.S. Pat. No.3,872,466 issued to Ivar Wold on Mar. 18, 1975.

For other applications, there has been widespread use of so-calledsuccessive-approximation converters. Such converters include adigital-to-analog converter (often called a DAC) which during theconversion cycle is sequenced through a predetermined algorithm wherebythe DAC output at appropriate stages is compared with the analog inputsignal to determine whether a corresponding bit of the final digitaloutput signal should be "high" or "low". The results of thisdetermination are used to set the respective stages of thesuccessive-approximation register (SAR). The nature of such operation isin general well known, and is described for example at Page II-81 of the"A D Conversion Handbook" published by Analog Devices, Inc. of Norwood,Mass. Still further information on various converters may be found inthe book "Electronic Analog/Digital Conversions" by H. A. Schmid (VanNostrand Reinhold, 1970).

Among the important requirements for interface devices such asanalog-to-digital converters is that they be small in size andeconomical to manufacture. Although such an objective has been suitablyrealized in a-to-d converters of the ramp-signal integrator type byconstructing the converter as an integrated circuit on one or twomonolithic chips, there has been no comparable advance achieved in thedevelopment of successive-approximation converters. In part, this hasbeen due to the difficulty of using conventional processing technologyto place on one or two chips a complete DAC together with all of thecircuitry required for the successive approximation functions forcontrolling the DAC and storing the results of the successive analogcomparisons.

SUMMARY OF THE INVENTION

In accordance with an important aspect of the present invention, asuperior IC analog-to-digital converter of the successive-approximationtype is provided by diffusing the substrate in such a way as to producea composite of normal mode transistors and inverted mode transistors,i.e. I² L (integrated injection logic) transistors for carrying out theanalog-to-digital conversion operations. By such composite construction,as will be described in detail hereinbelow, it becomes possible to placeon a single relatively small monolithic chip all of the circuit elementsneeded for a complete successive-approximation converter. Thisrepresents a significant step forward, particularly since efforts tocombine ordinary bipolar linear circuits with logic using the linearparts in conventional format have produced configurations which areprohibitively large. However, it has been found that in accordance withthe present invention the entire converter function can be carried outby elements occupying only a relatively small area, and which can bemanufactured using processes very similar to conventional diffusionprocesses.

For further information on I² L processes and techniques, reference maybe made to "Design Considerations for Merged Transistor Logic(Integrated Injection Logic)" by Horst H. Berger, pps. 14-15 of theDigest of the 1974 IEEE International Solid State Circuits Conference.There are numerous other publications providing additional informationon this subject.

Accordingly, it is an object of this invention to provide asignificantly improved analog-to-digital converter of thesuccessive-approximation type. Still other objects, aspects andadvantages of the invention will in part be pointed out in, and in partapparent from, the following detailed description considered togetherwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram, partly in block format, showing oneembodiment of the present invention comprising a completeanalog-to-digital converter formed on a single monolithic chip;

FIG. 2 shows diagramatically the positional relationship for combiningFIGS. 3 through 6;

FIGS. 3 through 6 together present a detailed schematic of theembodiment of FIG. 1;

FIG. 7 shows certain details of the successive-approximation registerwhich were omitted from FIG. 4 for the sake of simplicity; and

FIG. 8 is a simplified schematic illustrating the I² L/linear interfacefor two-bits of the DAC.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

A presently preferred analog-to-digital converter in accordance withthis invention comprises a single substrate which is subjected to amulti-step diffusion process to form a monolithic chip having in certainregions thereof, inverted mode I² L transistors. FIG. 1 shows thisembodiment diagrammatically. In this Figure, and also in FIGS. 3-6, theinverted mode transistors are located in an I² L "pocket" portion of thesubstrate, indicated by a dashed-line block, and the individualtransistors thereof are signified by emitters with a half-arrowhead; allof these emitters have a common connection to the logic return. The I² Linjection rail R is shown as a so-labelled, filled-in arrowhead. Theremaining portions of the chip comprise (with one minor exception)normal-mode transistors which are therefore not specially identifiedsymbolically.

The composite chip in accordance with the invention is made by adiffusion process having only one more step than the standard process,that of diffusing a deep n⁺ to prevent cross-talk between adjacent gatesin the structure. The process produces all of the standard components inaddition to I² L transistors. This can be achieved in an essentiallystandard process because in making the I² L gate device (1) the normalemitter diffusion forms the I² L collectors, (2) the bases are the sameand are formed simultaneously, and (3) the epitaxial region normallyused for the collector serves as the I² L emitter. Since eachmulti-collector I² L transistor is a self-contained logic gate, thepacking density can be substantially improved.

Referring now to the lower right-hand corner of FIG. 1, the chipincludes a 10-bit current-output DAC 30 (shown within an interruptedline block) and comprising a plurality of normal-mode transistor currentsources 32A, 32B, etc. The currents produced by these sources arebinarily weighted by means of a resistive network 34 connected to theemitters of the transistors. Each current source is controlled by arespective switch 36A, 36B, etc., comprising a differential pair oftransistors arranged to divert the source current either to a digitalcommon line 38 or to a current summing line 40, in accordance with thestate of the control signals supplied to the bases of the differentialpair switches. A DAC of the type shown herein is described in detail inU.S. Pat. No. 3,940,760, issued to A. P. Brokaw on Feb. 24, 1976.

The switches 36A, etc., of the DAC 30 are operated by asuccessive-approximation control means, generally indicated at 42, whichcarries out a conventional successive-approximation algorithm such as iswell known in the art of analog-to-digital converters. During executionof this algorithm, DAC output currents on line 40 are compared to theanalog input signal which is fed through the analog input terminal 44and an input resistor 46 to a summing point 48 which is connected to acomparator 50. The results of the comparison are directed through a line52 to the successive-approximation control means 42 to determine thestate of the individual bit flip-flops 54A, etc., forming part thereof.

In brief, the DAC 30 is controlled by the successive-approximationcontrol means 42 in such a way that the most significant bit (MSB)source 36A first is turned on, and its value is compared to the inputanalog signal. If the analog input is larger, the Bit 1 flip-flop 54A isset, and the MSB current source is maintained on by the associatedcontrol circuitry. Thereafter, the next current source 32B is turned on(having half the value of the MSB current), and another comparison ismade between the analog input signal and the combined currents of thefirst two current sources 32A, 32B. If the analog input signal isgreater, the Bit 2 flip-flop 54B is set, and the current source 32B isthereafter maintained in its on state by the associated controlcircuitry. If the analog input is smaller, the Bit 2 flip-flop is placedin reset state, and the current source 32B is turned off. This procedurecontinues in the well known manner until all current sources have beentested and compared with the analog input signal. The final state of theflip-flops 54A, etc., represents the digital number corresponding to theanalog input signal. These flip-flops are connected to respectivethree-state output buffers 56A, etc., which are activated at the end ofthe conversion cycle so as to produce a digital output signal on the bitoutput terminals 58A. etc.

A conversion cycle is initiated by applying a start signal to the "Blank& Convert" terminal 60. This start signal first goes high to produce the"blank" function, wherein the converter circuitry is reset to an initialcondition, and thereafter goes low, to begin thesuccessive-approximation conversion sequence.

Referring now to FIG. 3 as well as to FIG. 1, in the "blank" mode withB&C high (above the threshold level at the base of Q322), current fromQ320 is conveyed by Q322 to the base of Q341. This current drives Q341on which in turn drives on Q137. (Schematically, Q137 is shown in FIG. 3as a single, multiple-collector transistor; in the actual chip, it iscomposed of a number of one and two collector transistors which areconnected in parallel so as to act like a multiple-collector deviceresponding to the drive signal from Q341.)

As Q137 comes on, it begins to steal the base drive from Q341 by way ofone of its collectors returned to the Q341 base. Q341 is a normal modetransistor with relatively high β so that it requires only a small basecurrent in order to deliver enough drive to turn on Q137. As Q137 comeson, an equilibrium is reached wherein the collector current of Q137rises to equal the collector current of Q322, less only the small basecurrent of Q341. Bias circuits, consisting of Q139 (a current limiter),Q340, Q329 and Q328 establish this current at a level which is somewhatgreater than the base current of any injected transistor in the I² Larray. As a result, Q137 is turned on with sufficient drive to insurethat its collectors can sink the base current of any transistor in thearray.

(Note: To simplify FIG. 4, only the circuitry for the first two and thelast two bits is shown. The output buffers and the DAC current sourcesfor the other six bits are identical to those shown. To complete thepresentation, FIG. 7 has been included to show the details of thecontrol circuitry for the other six bits.)

One collector of Q137 drives the base of Q126 and thereby presets thecondition of a clock 62 the operation of which will be describedhereinbelow. Referring also to FIG. 4, the other collectors of Q137drive points in the successive-approximation control logic 42 toestablish the initial conditions of all of the bit flip-flops 54A, etc.

After all of the converter circuits have been cleared to initialcondition, the B&C input will be driven low. This causes the currentfrom Q320 to be diverted from Q322 to Q321 which drives the base ofQ125. Lacking base drive, Q341 switches off and the collector of Q125speeds the turn-off of Q137. When Q137 is off, its collectors releasethe successive-approximation control logic and also release the clamp onQ126 in the clock to initiate a conversion.

The internal clock 62 is a ring oscillator consisting of Q123, Q124,Q126-Q129 and Q131. Since the ring has an odd number of transistors (7),it is unstable, and oscillates at a frequency determined by the logicsignal propagation times. This arrangement minimizes problems due toprocessing-related variations in propagation delay since the clock speedchanges with the inherent logic speed.

The main clock loop drives two functions. One provides the comparatorlatch/sense signals on lines 63, 64; the other operates a divide-by-twoflip-flop 66 the output of which drives the successive-approximationcontrol and register 42 (hereinafter the "SAR") through its algorithm.The SAR advances on both rising and falling edges of the clock drive itreceives from the flip-flop 66. As a result, the SAR runs at twice theapparent clocking frequency. The divide-by-two flip-flop, composed ofQ130, Q132-Q136 and Q342 halves the clock frequency so that the SARadvances one step for each full cycle of the clock 62.

The I² L transistors in this divide-by-two circuit make up a D flip-flopwith the Q output tied back to the D input. The normal mode transistorQ342 is a buffer which drives the base of Q136. Drive for Q342 isderived from the same bias circuitry 67 as the B&C control current, i.e.transistors Q139, Q340, Q329 and Q328 as shown in FIG. 3. The drive iscontrolled by Q134 and Q135 which would drive Q136 directly in aconventional D flip-flop. The buffering, supplied by Q342, is needed todrive the multiple collectors of Q136. These collectors control theclocking of the SAR. (As with Q137, the schematic of FIG. 3 shows Q136as a single multiple-collector device although it is actually a numberof individual transistors driven by a common base line.)

Adequate drive is insured by supplying the base of Q342 with somewhatmore than the normal I² L gate current derived from the injector rail R.When Q342 drives Q136 on, Q136 must sink more than the normal gatecurrent before reaching equilibrium with the current from Q328. As aresult, Q136 is driven hard enough that the other collectors can allsink at least the maximum gate current.

The current from Q328 is limited, however, so that the base voltage ofQ342 can be controlled by the double-sized collectors of either Q134 orQ135. These drive transistors also have a collector driving the base ofQ136 to remove excess charge when Q342 switches off.

The circuit described provides a clock drive serving all of the SAR 42,and which alternates once each time Q131, in the main clock oscillator,switches off. Use of the D flip-flop 66 to halve the frequency resultsin a lower transistor count than would be required to achieve the samefrequency with a ring oscillator of twice the number of stages. Also,use of frequency division allows the comparator to be latched justbefore each advance of the SAR. Since the SAR advances on both leadingand trailing edges of its clock drive, the comparator must be latched(and unlatched) at twice the SAR clock frequency. The comparator istherefore operated from the ring oscillator loop and the SAR is operatedfrom the ring oscillator frequency divided by two.

In the diffused chip, the bases of Q127 and Q131 are enlarged topartially surround their respective injector region. As a result, theyhave an excess of base drive, some of which is used to supply the basecurrents of Q355 and Q356 in the comparator 50. These last twotransistors control one of the main bias circuits in the comparator.When Q355 is on, current from Q372 drives the comparator input stage andthe comparator operates in a linear mode. If Q356 is switched on andQ355 is switched off, the current from Q372 is by-passed around theinput stage and through a flip-flop circuit connected with thecomparator, and which latches in the current state of the comparator.Since under that circumstance the input stage is disabled, subsequentchanges in the comparator input signal will have no effect and thecomparator output will remain latched with the information present atthe switching time. The comparator can be restored to linear, orinput-sensitive operation, simply by reversing the drive to the controltransistors, switching Q356 off and Q355 back on again.

When the B&C signal goes low to "convert" and causes Q137 to release thebase of Q126, an inversion begins to propagate around the ringoscillator of clock 62. The base of Q126, which was low, rises and Q126turns on driving the base of Q127 low. When Q127 goes off, Q131 isallowed to go on, and the inversion continues around the ring. With thebase of Q127 held low and the base of Q131 high, the comparator is inthe input-sensitive mode and senses the difference between the MSB andthe analog input signal.

When the inversion completely circles the ring, Q127 will be switched onand it will, in turn, switch off Q131. These two base signals will causethe comparator to latch the state of its output. As Q131 goes off, twoof its collectors trigger the flip-flop 66 and cause it to toggle,advancing the state of the SAR. When the SAR advances, a new testcondition is presented to the comparator. However, the results of theprevious test will remain latched into the comparator until theappropriate switching is complete. The inversion which triggered the Dflip-flop 66 will continue around the ring until it reaches Q127 andQ131 again, whereupon the comparator will be restored to theinput-sensitive mode to examine new test condition, and the flip-flopinputs will be driven low to arm it for the next clock pulse.

Test Sequence of the Successive-Approximation Register

The individual bit current sources 32A, etc. of the DAC 30 arecontrolled by the flip-flops 54A, etc., in the SAR. Each of theseflip-flops consists of a pair of transistors: Q2 and Q4 for the MSB; Q12and Q14 for the 2nd SB; Q82 and Q84 for bit 9, etc., and in general Q×2and Q×4 for bit ×+1. Each of the bits is sequentially tested beginningwith the MSB and progressing to the LSB. The SAR circuitry repeats on a2-bit cycle inside the register. The beginning and end (MSB and Status)of the register are modified slightly from the cyclic pattern toaccommodate their start and finish functions.

The resetting function of the B&C input, which is implemented by Q137,clears the flip-flops associated with bits 2 through 10, so that thesebits are not expressed in the DAC output. The MSB is switched on by acollector of Q137 which drives the base of Q2. When a conversion begins,Q137 is switched off leaving the 10 flip-flops in their cleared state,but capable of being flipped by other signals.

The reset functions of Q137 also clears 5 control flip-flops 68A/B . . .68I/J, one for each adjacent pair of bit flip-flops 54A, 54B; 54C, 54D;etc., and consisting of pairs of transistors Q27-Q28 . . . Q107-Q108.These flip-flops control the sequence of events in the SAR. Inparticular, Q27 is on after the reset signal from Q137 and holds off Q31(FIG. 7). Q51 is held off by Q47 and so forth to Q91 which is held offby Q87. There is no preceding control flip-flop to hold off Q11 (whichis similar in function to Q31, Q51, etc.). However, when the convertsequence begins, Q136 is on, holding all SAR clock lines low. During thefirst clock interval, while the base of Q11 is held low, the Q2-Q4flip-flop is on alone and the MSB is tested. At the end of the firstmain clock cycle, the D flip-flop 66 is switched and Q136 goes offallowing Q11 to come on. When Q11 comes on, it switches Q12 off causingthe Q12-Q14 flip-flop to toggle. Outputs from Q12 and Q14 switch on thesecond bit of the DAC 30 by way of Q217 and Q218. During the intervalfollowing this transition, the second bit is tested.

In the reset state, Q12 holds Q21 off. When Q11 comes on, switching Q12,Q11 holds Q21 off. At the end of the high clock cycle at the base ofQ11, however, Q11 is switched off, Q12 has been previously switched off,and Q21 is allowed to come on. When Q21 comes on, it drives Q27 offthereby toggling the Q27-Q28 flip-flop. Since this flip-flop can becleared only by the general B&C reset, it will remain set for theremainder of the conversion. Outputs from Q28 now go low blocking anysubsequent operation of Q11, Q21, Q3 or Q13. Once Q28 comes on, itinsures that the preceding portions of the successive-approximationsequence will not be repeated in this cycle.

When Q21 comes on it also drives Q22 off (FIG. 7) setting the Q22-Q24flip-flop and initiating the testing of bit 3. This test will continuewhile the collectors of Q136 remain low.

One additional change during this interval is the enabling of Q31. WhenQ27 is switched off, it releases the base of Q31 which, however,continues to be held low by a collector of Q136. When this clock cycleends and the D flip-flop 66 turns off Q136, Q31 will come on setting theQ32-Q34 flip-flop. This situation is analogous to the sequence initiatedby the clocking of Q11. Subsequent operations are also analogous. Thatis, bit 4 is tested while Q136 is off. When Q136 is next switched on,Q41 comes on setting both the Q47-Q48 and Q42-Q44 flip-flops. Bit 5 willbe tested, the preceding circuits will be blocked by Q48, and Q51 willbe enabled by Q47. Each succeeding rise and fall of the clock signalsfrom Q136 will advance the test bit by one position until all 10 bitshave been tested.

At the end of the test interval for the tenth bit, the collectors ofQ108 "jam" preceding sections of the register. Also, one collector ofQ108 is returned to the base of Q126 to stop the clock 62 after 10 bitshave been converted. One collector of Q107 drives Q109, a controlled βI²L transistor comprising a half-size current-limiting collectorconnection to its base. This transistor switches on and draws a currentwhich approaches twice the gate current of a single injected device.This current drives the status buffer 70 by way of Q302.

The status buffer 70 indicates that a conversion is complete, and italso drives, via a line 71, the 10-bit three-state output buffers 56A,etc., into the indicating state. Each of the bit output buffers isdriven by a collector from the DAC control flip-flop (Q4, Q14, Q×4) toindicate the final state of the DAC. This state will be within one bitof balancing the input signal (for signals inside the converter's range)and so indicates digitally the magnitude of the analog input.

Comparator Control

The description of the test sequence above shows how each bit of the DAC30 is switched on as the conversion proceeds. In order for the DACoutput to converge on a value approximating the analog signal input,means must be provided to switch off bits which, when summed withpreviously selected bits, exceed the input signal. The comparatorcircuit detects the sign of the difference of the analog input and theDAC output. Its output through line 52 drives the base of Q138 with theresult of the comparison, and the output is latched on or off by theclock 62 during a particular period of the clock cycle.

When the analog input exceeds the DAC output, Q138 will be driven on;when the DAC output exceeds the analog input Q138 will be driven off.FIG. 3 indicates that Q138 is a single multiple-collector device, butlike the clock and reset transistors Q136 and Q137, it is actuallycomposed of a number of transistors driven in parallel. Unlike the clockand reset transistors, however, it does not use feedback control of itsdrive. Instead, the base connection to Q138 is overdriven by thecomparator in the on state.

The collectors of Q138 control the transistors Q3, Q13 . . . Q×3associated with the bit flip-flops 54A, etc. These transistors Q3, etc.,are arranged so that they will reset their associated flip-flop at theend of the test interval unless they are inhibited by Q138.

Consider the action of Q3 on the Q2-Q4 flip-flop. During the test of theMSB, the collectors of Q136 are low. One of these collectors drives Q3and prevents it from resetting Q2-Q4. At this time both Q14 and Q28 areoff so that at the end of the MSB test interval when Q136 goes off, Q3will come on and reset the Q2-Q4 flip-flop unless the collector of Q138driving Q3 inhibits it.

When the clock transistor Q136 goes off, the previously describedsequence where Q11 sets the Q12-Q14 flip-flop begins. Therefore, afterthree gate delays, Q14 comes on and inhibits Q3 from resetting the Q2-Q4flip-flop. That is, when the clock collectors go high at the end of theMSB test interval, Q3 is enabled for a period of three gate delays, toreset the Q2-Q4 flip-flop unless it is inhibited by the comparator byway of Q138. Subsequently, when the clock transistor is driven back on,Q14 may be reset. However, Q28 will come on and remain on for theremainder of the conversion to inhibit Q3. Since the clock collectorsare low during this change, any underlap between Q14 and Q28 is maskedby Q136 at the base of Q3. Therefore, Q3 has only one possibleopportunity during the entire conversion to reset the Q2-Q4 flip-flop.

When the collectors of Q136 go high to initiate testing of the secondbit, Q11 is driven on setting the Q12-Q14 flip-flop and inhibiting Q13from resetting it. At the end of the bit 2 test interval, the clockcollectors go low switching Q11 off and removing its inhibiting effecton Q13. If Q13 is not inhibited by its drive from the collector of Q138,it will come on and reset the Q12-Q14 flip-flop. When Q11 goes off,enabling Q13, it also initiates the chain of events which drives Q21 on,Q27 off and Q28 on. One collector of Q28 is connected to inhibit Q13 sothat Q11 enables Q13, and three gate delays later Q28 disables it again.Since Q28 will remain on for the entire remainder of the conversion, Q13is enabled to reset the Q12-Q14 flip-flop for only this three-gate delayinterval at the end of the second bit test. Depending on the state ofQ138, which is controlled by the comparator, the second bit will beretained or rejected at this time.

The function of Q23 in resetting the Q22-Q24 flip-flop is nearlyidentical to that of Q3 with respect to its flip-flop. During the secondbit test interval, Q23 is enabled. However, at that time the Q22-Q24flip-flop is still in its initial reset state so that Q23 has no effect.At the beginning of the fourth bit test interval, Q23 is enabled by theclock for the three gate delay interval necessary to switch Q34 on.During this enabled time, bit 3 is retained or rejected under control ofthe comparator. After this time and for the remainder of the convertcycle, Q23 will be inhibited by one or more of Q34, Q48, Q138 or Q136.The operation of Q33 in relation to bit 4 is identical to that of Q13 tobit 2. Each stage of the SAR has a reset transistor Q3, Q13, Q23, Q33,etc., which is enabled for three gate delays at the end of its testinterval so that the stage may be reset by the comparator asappropriate.

Q5, Q6; Q15, Q16; Q25, Q26; etc., as shown as collectorless I² Ltransistors on the schematic, are clamped current sources which supplythe drive for the DAC current diverters Q207, Q208; Q217, Q218; Q227,Q228; and so on. These transistors Q5, etc., are simply injected I² Lgates without collectors. Their unloaded voltage will rise to,approximately, the injector voltage and they can supply a normal I² Lgate current to a load. They are driven off, in alternation, by the bitcontrol flip-flops 54A, etc., so as to cause only one of a pair oftransistors Q2×7 and Q2×8 to conduct. This arrangement allows theflip-flop to divert the individual bit currents of the DAC to the analogsumming point or ground. This method is preferred to direct drive of thecurrent diverters by the flip-flop bases, so as to provide adequate basedrive for the high order bits, and to prevent unwanted dynamicinteraction between the DAC and the logic.

The circuitry of the comparator 50 provides for fast latching, in partbecause it includes as an integral element thereof an internal flip-flopcontrolled directly by the comparator currents. In more detail, thesense comparison is effected primarily by a pair of transistors Q351,Q352 the currents of which pass through respective load circuitscomprising R410, Q347; and R411, Q348. Transistors Q347 and Q348 areprovided with additional emitters which, when the clock 62 switchestransistors Q356 on, conduct current through that transistor. Whenswitchover occurs, the flow of current activates an internal flip-flopcomprising Q345, Q346 which thereupon latches the sensed state of thecomparison and fixes the output signal of the comparator at the latchedvalue.

The double-ended output signal from the comparator 50 is directedthrough respective circuits to a pair of level-shifting Zener diodes 73,74 diffused into the substrate together with respective transistor Q343,Q344. These transistors form a differential pair the collectors of whichare connected to a circuit Q331, Q370 arranged to convert thedouble-ended comparator signal to a corresponding single-ended signalfor the comparator output line 52.

The currents flowing through the comparator 50 and its output circuitryare controlled by conventional negative bias circuits indicated at 77.The summing point 48, at the input to the comparator, also is connectedto a bipolar offset current source in the form of a current mirror 75.The current of this source is controlled by a current developed by atransistor Q378 supplied by a voltage supply 76 which applies a supplyvoltage to a common base line 78. When the current mirror is activated,it supplies to summing point 48 a current equal to one-half thefull-scale current, thus providing the required offset effect to achievebipolar operation. The voltage supply 76 comprises a Zener diode D402which preferably is diffused to provide a sub-surface breakdown Zenerdiode, i.e. a so-called buried-layer Zener. The other Zener D401provides circuit start up.

The transistor base line 78 is connected to the current sources 36A,etc., of the DAC 30, and includes interbase resistors R451, 477, etc.,through which is directed a PTAT (proportional-to-absolute-temperature)current in accordance with the teachings of U.S. Pat. No. 3,940,760referred to above. The voltage reference 76 includes suitable circuitrycomparable to that described in said patent, to produce suchproportional current variation with temperature.

The voltage reference 76 also incorporates circuit arrangements toensure that the currents developed in the DAC output by the transistorcurrent sources 32A, 32B, etc., remain substantially constant in theface of changes in ambient temperature and supply voltage. Turning nowto this voltage reference circuit in more detail, the Zener diode D402is shown with two cathodes which indicate a force and sense, or Kelvinconnection to the junctions making up the diode. (For purposes ofunderstanding the circuit operation, these two cathodes can beconsidered as a single connection point, and as a practical matter thecircuit is operable as described with the two lines from resistors R429and R430 joined.) As the diode D402 goes into breakdown and begins toconduct, current flows in R430, and the upper end of R430 (and the leftend of R429) goes positive until Q362 turns on due to the positivebase-emitter voltage of Q362 across R430. Thus, the voltage at the topof diode D402 is stabilized at a voltage which is the sum of the V_(BE)of Q362 and V_(Z) for D402, plus a small voltage across R429 which willbe discussed subsequently.

The voltage at the top of the Zener diode D402 is applied through aresistor R432 to the voltage reference output which is connected to thebase line 78 of the D/A converter (DAC) previously described. Thisvoltage reference biases the R-2R ladder network, including resistorsR453, R455, R456, etc., so as to produce the binary weighted currents atthe converter output. The bias voltage is automatically controlled, aswill be explained, so as to stabilize the binary weighted currentsagainst the effects of changes in temperature and supply voltage.

One of the problems to be dealt with is the effect of the changes involtage of the Zener diode D402 with changes in temperature. Animportant element of the solution to that problem is provided byresistor R432 and associated circuitry. More particularly, it will beseen that the PTAT current for the interbase resistors R457, R477, etc.,also passes through resistor R432 to develop a voltage drop which variesdirectly with temperature, i.e. having a positive temperaturecoefficient (T.C.). Since this resistive voltage drop is subtractivewith respect to the voltage of the Zener diode D402, changes in theresistive voltage drop with temperature serve to compensate for theinherent positive T.C. of the Zener voltage.

Good compensation can be effected by selecting the value of resistorR432 and the characteristics of the PTAT current source such that theslope of the PTAT resistive voltage drop closely matches that of theZener voltage source, so that the resistive drop changes compensate forchanges in the source voltage. This result can particularly be achievedby laser-trimming of resistor R432 to the correct value. Alternatively,the resistor can be trimmed to compensate not only the voltage source,but also any other uncompensated temperature-responsive variations inoutput remaining after all other compensations have been effected, suchas will be described.

Turning now to such other compensations, a further problem is presentedby the fact that there are changes with temperature of the V_(BE) ofQ378 and the other current source transistors which terminate the R-2Rladder. Such V_(BE) changes effectively alter the bias voltage on theR-2R ladder, and thereby correspondingly tend to alter the bit currentsin the DAC output. To correct for this, the cathode of the diode D402 isconnected through a transistor Q362 to V- (i.e. the negative terminal ofthe reference supply) so that the voltage at the top of the diodecontains the V_(BE) of Q362. Thus, as the V_(BE) of Q378 (and the othercurrent source transistors) changes with temperature, the correspondingchange in the V_(BE) of Q362 will alter the voltage at the top of thediode D402 to provide compensation. By arranging the transistors Q362and Q378 to have at least approximately equal current densities (such asby use of the same number of emitters), and preferably equal densities,this compensation can be made suitably exact, thereby to maintain aconstant bias voltage on the R-2R ladder in face of changes intemperature. Since the voltages at all of the other DAC transistorcurrent source emitters are arranged to be equal to the emitter voltageof Q378, the voltage on the entire ladder is stabilized against theeffects of temperature on V_(BE). Such equal emitter voltages can beachieved in various ways, such as by using current sources with equalcurrent densities (e.g., as with Q378 and Q209), or by using interbaseresistors as shown in FIG. 5, and described in U.S. Pat. No. 3,940,760previously referred to herein.

To further improve the results with the compensation arrangement justdescribed, the current in Q362 preferably is stabilized by associatedcircuitry to be at least approximately constant. In the presentembodiment, this circuitry serves to make the current in Q367 at leastapproximately constant, and to provide that the current in R430complements that in R432, i.e. such that an increase in current throughR432 is accompanied by a commensurate decrease in current through R430.In this way, the sum of the PTAT current through R432 and the current inR430 will remain at least approximately constant, thereby leaving aconstant remainder of such constant current from Q367 to pass throughQ362. In the present embodiment, this result is enhanced by arrangingthe circuit so that the resistance of R430 times the sum of the currentsthrough R430 and R432 is at least approximately equal to the bandgapvoltage of the material.

The current through Q367 is stabilized by an amplifier including Q368and Q369. These transistors drive the bases of Q366 and Q367 so that theQ366 current through resistor R445 will produce at the base of Q368 avoltage equal to the voltage at the top of the Zener diode D402. SinceQ367 parallels Q366 and has twice the emitter area, it puts out twicethe amount of current into the Zener circuitry previously described.Since the Zener diode D402 has an inherent positive T.C., and the V_(BE)of Q362 has an inherent negative T.C., the voltage at the top of thediode D402 is (approximately) temperature-compensated. Consequently,since the voltage across R445 is driven by the current from Q366 tomatch the Zener diode voltage, the current from Q366 (and thecorresponding current from Q367) is in turn temperature-compensated.

The output impedance of Zener diode D402 coupled with Q362 is very lowand the impedance of Q367 is high, so that the resulting bias circuit isquite insensitive to supply voltage variation.

Resistor R429 between diode D402 and Q362 provides an additionaltemperature compensation effect. The base current of Q362 flows in R429and produces a voltage drop inversely proportional to β. This additionaldrop is translated to the top of the diode D402 to appear ultimatelyacross the R-2R ladder, resulting in a slight increase in each of theDAC bit currents. Changes in the voltage drop across R429 withtemperature tend to compensate for changes in the DAC output current dueto changes in β of the current source transistors with temperature.

In more detail, of the binary weighted currents in the DAC, it will beseen that a small fraction is diverted away from the output due to thebase current of Q378 and the other transistors terminating the ladderthrough Q309. Still other current is lost due to the base currents ofQ208, Q218, etc., serving as the current source switches. This divertedcurrent is a function of β and therefore varies with temperature. Thechanges in ladder voltage due to the variation in base current throughR429 tend to compensate for these changes in such diverted current, and,since both changes are β-related, they will track in proper proportion.By selecting the value of R429, the additional voltage drop producedacross the ladder will add a current to each ladder output which equalsthe amount lost to base currents in the DAC control and switchtransistors.

Accordingly, it will be understood that the reference-voltage apparatusdisclosed in FIG. 5 of the drawings serves to bias a current-output DACdirectly from a Zener diode without the need for an interveningamplifier. The compensation and bias circuits are arranged to produce astable voltage directly across the binary weighted network, and afurther compensation is provided for base currents.

Biasing

In addition to generating the various bias currents and voltages used inthe converter, and mentioned elsewhere, the biasing of the I² Lcircuitry is specially arranged to take maximum advantage of existingbias levels and to avoid the need for level translator structures. TheI² L circuitry is connected so that the injector R, which is its mostpositive power terminal, is driven from a bias voltage which is negativewith respect to ground. This low impedance bias is generated by theforward conducting voltage drops of Q323 and Q324 acting as diodes. Theswitches 36A, 36B, . . . etc., must be negative with respect to groundin order to drive the summing point 48 of the comparator 50 so as toconverge toward ground potential. The negative bias voltage of thesuccessive-approximation control 42 permits it to directly drive theswitches without voltage level translators, which would be requiredusing conventional logic power voltages.

Moreover, because it is the positive power connection to thesuccessive-approximation control logic 42 which operates at fixedvoltage (two diode-drops below ground), the I² L circuitry can bepowered in part by currents derived from operation of the Zener supplyvoltage source 76 and the DAC circuits. This current is used by thesecircuits, which approximately regulate its amplitude, and they havesufficient voltage compliance to permit them to be connected to thenegative side of the logic. In this way these currents are used to powerthe I² L transistor circuitry as they are returned to ground. Thispermits a saving of overall power over the conventional approach whereinthe negative power connection is fixed by a low impedance connection andadditional fixed current would be used to power the positive logicconnection.

The negatively biased I² L circuitry must drive the output buffers 56A,56B, . . . etc., which are biased between ground and the positive supplyfor compatibility with external circuitry. Connection between the I² Llogic and the buffers is accomplished by driving a single inverted modeNPN collector which has sufficient voltage compliance to accommodate thedifference in bias voltage levels. This represents no increase incomplexity over a conventional biasing arrangement.

The inventive aspects described immediately above can perhaps be betterunderstood through reference to the simplified schematic of FIG. 8illustrating the I² L/linear interface for two bits of the converter.This schematic shows how the I² L logic directly drives the differentialswitches (Q207, Q208, etc.) in the DAC, thereby eliminating the buffersusually required between such elements. The switches are supplied byrespective current sources (Q209, Q219) which terminate the R-2R ladderproducing the binary weighted currents. The low voltage swings of the I²L are ideal to drive the NPN differential switches which divert thebinary weighted currents either to the DAC output or the digital commonline.

Since the DAC output is driven to a null around analog common, thedifferential switches are negative with respect to analog common. Thelogic array is biased negatively by the diodes Q323, Q324 so that thelogic level is appropriate for driving the switches. This level alsoallows the I² L collectors to directly drive the 3-state digital outputgates. The negative currents activating the buried Zener reference diodeD402 are reused to provide power to the I² L logic array.

Contrary to standard practice, in the disclosed embodiment the I² Llogic is powered from a fixed positive voltage (specifically, as shown,two diode drops below ground) and driven by a negative current. Thisnegative current is that used first by the voltage-reference circuitry,plus a small amount of current derived from Q377, shown in FIG. 5. (Useof the latter transistor is however not a requirement.) It particularlyis important to note that this biasing arrangement is a reversal of theusual practice which is to ground the negative terminal of the I² Lelements and supply operating current to the positive terminal.

Although a preferred embodiment of the invention has been describedherein in detail, it is desired to emphasize that this is for thepurpose of illustrating the principles of the invention, and should notnecessarily be construed as limiting of the invention since it isapparent that those skilled in this art can make many modifiedarrangements of the invention without departing from the true scopethereof.

We claim:
 1. A monolithic integrated circuit chipincluding:digital-to-analog converter (DAC) means comprising a pluralityof transistor current sources each including base, collector and emitterelectrodes; an output terminal for said converter means to provide anoutput current representing a summation of the individual currentscontributed by said current sources in accordance with an input digitalsignal; resistance network means connected to the emitters of saidcurrent source transistors to control the levels of the currentscontributed thereby to said output terminal in accordance with aselected weighting pattern; said network means including a biaspotential terminal; said digital-to-analog converter including meansarranged so that the potentials of said current source emitters aremaintained equal under different operating temperature conditions; acommon base line including circuit means coupling the bases of saidtransistor current sources together to provide for potential trackingthereof; voltage reference means to furnish bias potential for saidcurrent sources and said resistance network means, and including a pairof output terminals; means connecting said output terminals respectivelyto said common base line and said bias potential terminal; said voltagereference means comprising a junction providing a d-c supply voltage themagnitude of which varies with the temperature of said chip; resistormeans connected between one side of said junction and the correspondingvoltage reference output terminal to produce a voltage drop thereacrossin accordance with the magnitude of current flowing through saidresistor means; current supply means to provide a flow of currentthrough said resistor means to produce said voltage drop thereacross;and said current supply means including current control means to causesaid flow of current to vary with temperature in direction and amount toprovide temperature compensation for the effects of said variations ind-c supply voltage of said voltage reference by producing correspondingchanges in said voltage drop.
 2. An IC chip as in claim 1, wherein saidcurrent supply and control means comprises means to control said flow ofcurrent so that its magnitude is proportional to absolute temperatureand in a direction to develop a corresponding subtractive voltage dropreducing the reference voltage appearing at said output terminals.
 3. AnIC chip as in claim 2, wherein said DAC means current-source emittersare set at equal potentials by electrical resistance means connectedbetween the bases of at least certain of said current source transistorsto carry the current from said current supply means, thereby to providefor temperature compensation of the emitter potentials to maintain themequal.
 4. An IC chip as in claim 1, including a compensating transistorcurrent source having base, emitter and collector electrodes; andmeansconnecting the base-to-emitter voltage of said compensating currentsource in series with said voltage reference means junction.
 5. An ICchip as in claim 4, wherein said compensating current source is arrangedto have a current density at least approximately equal to the currentdensity of the one current source of said DAC means providing themaximum current contribution.
 6. An IC chip as in claim 4, includingbase resistor means connected between said junction and the base of saidcompensating transistor, to provide a voltage drop varying with basecurrent so as to compensate for the effects of changing base currents onthe current contributions of said DAC means current sources.
 7. An ICchip as in claim 4, wherein the collector of said compensatingtransistor is connected to one side of said junction and the basethereof is connected to the other side of the junction;thebase-to-emitter voltage of said compensating transistor varying withtemperature in such fashion as to at least approximately compensate forthe temperature-responsive changes in junction voltage, so as to produceat least approximately constant potential at said one side of saidjunction; said current supply means furnishing current to (1) saidjunction, (2) said collector of said compensating transistor, and (3)said resistor means; and second current control means for controllingthe total current of said current supply means in accordance with saidjunction potential whereby said total current is at least approximatelyconstant.
 8. An IC chip as in claim 7, arranged to provide that saidjunction current and said current through said resistor means arecomplementary such that a temperature-responsive increase in one isaccompanied by a temperature-responsive decrease in the other, wherebythe current through said compensating transistor is maintained at leastapproximately constant with temperature.
 9. An IC chip as in claim 8,including a resistor connected in series with said junction to carry thejunction current;the chip elements being so arranged that the resistanceof such resistor, multiplied by the sum of the junction current and thecurrent through said resistor means, is at least approximately equal tothe band-gap voltage of the chip material.
 10. An IC chip as in claim 1,wherein said junction forms part of a Zener diode voltage source.
 11. Ananalog-to-digital converter comprising a single monolithic chip formedwitha plurality of normal mode transistor current sources arranged toproduce respective binarily-weighted currents for summation into acomposite signal; I² L inverted mode transistor means including logiccircuit means defining successive-approximation control means arrangedto carry out a successive-approximation algorithm; switch meanscomprising a series of individual switch circuits serving to selectivelycontrol the outputs of said current sources; clock means for supplyingclock pulses to said logic circuit means to carry out saidsuccessive-approximation algorithm; said successive-approximationcontrol means comprising a series of register stages which are activatedin sequence as the algorithm proceeds; each of said individual switchcircuits being operable by a respective one of said register stages andconnected to a corresponding current source, said switch circuits beingoperable when switched by the respective register stage to cause currentfrom its source to be supplied to a summing point, whereby thecombination of selected currents develops at said summing point saidcomposite signal for comparison with an analog input signal; andtemperature-stable voltage reference means formed on the same chip andinterconnected with said analog-to-digital converter to provide anabsolute analog-to-digital converter.
 12. A converter as in claim 11,wherein said voltage reference means comprises a buried-zener reference.13. A converter comprising a single monolithic chip formed with:aplurality of transistor current sources with their bases interconnectedand arranged to produce currents for summation into a composite signal;a resistor network having an input terminal and a plurality of otherterminals connected to the emitters of said current sources; a voltagesource having a positive TC; means coupling the output of said voltagesource between the base of one of said transistor current sources andsaid network input terminal; resistor means connected in said couplingmeans for producing a voltage drop; and current source means producing aflow of PTAT current through said resistor means to compensate for saidpositive TC.
 14. Apparatus as claimed in claim 13, including interbaseresistors connected between the bases of said transistor current sourcesand producing voltage drops responsive to said PTAT current. 15.Apparatus as claimed in claim 13, wherein said voltage source is a Zenerdiode.
 16. Apparatus as claimed in claim 13, wherein one terminal ofsaid voltage source is coupled to said one transistor base; andacompensation transistor having its base coupled to the other terminal ofsaid voltage source and its emitter coupled to the power supply line;said compensation transistor having a current density at leastapproximately equal to that of said one transistor current source, toprovide for V_(BE) compensation thereof.
 17. Apparatus as claimed inclaim 16, including means to maintain the current through saidcompensation transistor at least approximately constant.
 18. Apparatusas claimed in claim 16, including resistor means connected to the baseof said compensation transistor to produce a voltage drop compensatingfor variation in β of said one current source transistor.
 19. Apparatusas claimed in claim 13, including means to compensate for variations inβ of said one current source transistor.